From the aforementioned copending application it will be apparent that a process utilizing pressure-swing adsorbers for the cleaning and rectification of gases, especially utilizing a plurality of adsorbers, is known in the art with the important advantage that thermal regeneration of the adsorber gas is not required.
An installation for the purposes described can comprise a multiplicity of adsorbers which undergo, in each operating cycle, a predetermined number of adsorption phases, pressure-relief or pressure-equalization phases, purging phases and representation or pressure buildup phases.
For the cleaning and rectification of gases, especially the cleaning of natural gas or the rectification of noble or inert gases, air, sewer gases, cracking gases, hydrogen-rich gas mixtures or mixtures of gaseous hydrocarbons, it is a common practice nowadays to utilize adsorption processes because of the high economy and effectiveness.
In recent years, this field has seen an increased rate of growth because of the introduction of quasi-isothermal processes which are generally referred to as pressure-swing adsorption processes.
In these processes, the desorption or regeneration of the charged or loaded adsorption medium is not effected by an increase in temperature which must be followed by a cooling to the adsorption temperature, but rather exclusively by a reduction in pressure over the loaded adsorbent, i.e. a pressure relief of the adsorber, whereby the desorption stage is effected at least during its later phases by the passing of a purge gas through the medium.
It is also known, in this pressure-swing adsorption technique, to utilize the gas freed upon the pressure relieving of a loaded adsorber, more or less completely to increase the pressure (during respective pressure build-up phases) in other absorbers which have previously been brought to low-pressure levels by the respective pressure relief phases.
For example, German patent document (open application-Offenlegungsschrift) DE-OS 26 34 346 describes a process for the cleaning or rectification of an input gas mixture by pressure-swing adsorption units totaling nine in the installation. Each of these adsorbers is assigned six valves.
Following the adsorption phase at high pressure, each adsorber, in turn, undergoes a plurality of pressure-relief stages and the gas initially found in the interstices of the adsorption agent and later also adsorbed components, is withdrawn and in part used for pressurization of adsorbers which have been brought to their low pressure levels and must be repressurized.
The first pressure-relief stage is effected by connecting the adsorber in question with another adsorber already at the lower pressure level characterized by the first pressure-relief stage, i.e. by equalization of pressure between the two adsorbers. Naturally, the adsorber initially at the lower pressure thereby attains a higher pressure as part of its repressurization stage.
In this manner each pressure-relief stage and each repressurization stage can be effected by pressure equalization between adsorbers.
In the last pressure-relief stage, gas is withdrawn as residual gas and this pressure-relief stage can be followed by a counterflow purging of the adsorber to eliminate as best as possible the adsorbed components from the adsorptive medium.
The subsequent repressurization phases are then carried out from the most part by pressure equalization with other adsorbers at high pressure with the last repressurization stage effected to the adsorption pressure with product gas.
In such installations, the durations of the individual phases within a cycle may be of the order of seconds, while the total cycle between readiness for the adsorption phase to a repetition of such readiness, may be of the order of several minutes.
As a result of the high speed at which the pressures and gas flows must be changed over for the numerous phases of each cycle and between adsorbers, the system can be deemed to be under considerable stress. In other words, the high sequencing and alternation speeds place significant strain upon the operating members such as valves, and even static members such as conduits and fittings. The problem is increased by the practical requirement that the system operate without major replacement or modification for periods of up to ten years in a more or less continuous manner.
When such systems are constructed for long-term operation, it is found that despite the most effective choice of design, various failure problems arise primarily as a result of material fatigue or wear. As a result, adsorbers, valves and even the ducts connecting the adsorbers with one another, the valves with one another, or the adsorbers with the valves, can fail and result in detrimental conditions within the system manifested, for example, by pressure losses and leakage. For example, the valves, ducts or adsorbers may leak pressurized gas to the environment or valves may leak pressurized gas into unpressurized regions, etc.
In conventional installations of the aforedescribed type it is frequently required to bring the entire installation to standstill so that the components can be checked and tested, effects ascertained and failures replaced, repaired or modified before the system is again placed in operation. The downtimes which this approach necessitates are significant economic disadvantages but are even more detrimental than can be measured by the loss of operating time, since installations of the type described are frequently provided in adsorption systems in which the recovered fractions of the original or incoming gas must be obtained in substantially constant quantities practically continuously for other units making use of these fractions or for distribution to consuming installations.